1. Field of the Invention
Generally this invention pertains to a wavelength reference and calibration device, and more specifically to a fiber Bragg grating interrogation system for determination of Bragg grating wavelengths.
2. Description of the Related Art
There is a need for accurate measurement of Bragg gratings wavelengths that includes long-term and static strain monitoring on structures and determination of wavelengths in optical communications systems. There a number of systems that use the wavelength of fiber Bragg gratings to indicate the value of a measurand such as strain or temperature often at distributed points on a structure. In systems of multiple Bragg gratingsxe2x80x94especially in a single fiberxe2x80x94bandpass filters that scan through a range of wavelengths are commonly employed. (SEE, Kersey et al.; A MULTIPLEXED FIBER BRAGG STRAIN SENSOR SYSTEM WITH A FIBER FABRY-PEROT WAVELENGTH FILTER; Optics Lett., Vol/18, Pg. 1370, 1993.) In some schemes, the control signal applied to the filter is used to determine the wavelength of the individual gratings. This practice depends on an estimated functional relationship between the filter control input and the wavelength location of the passband, a function that is not in practice linear or constant in time. For dynamic measurements a one-time calibration is often adequate, while for very low frequency measurements a real-time calibration is necessary for accurate determination of Bragg grating wavelength. Current filter calibration options include wavelength references such as temperature-isolated gratings (SEE, U.S. Pat. No. 5,818,585) or the fringe pattern of a temperature-isolated Fabry-Perot cavity (SEE, U.S. Pat. No. 5,892,582). In either case, the reference wavelengths are sensitive to changes in temperature and care must be taken to keep the gratings or cavity at a constant temperature.
The object of this invention is to provide an interrogation system for fiber Bragg gratings enabling accurate determination of Bragg grating wavelengths that is compensated for changes in temperature, if required.
This and other objectives are met by a passive, temperature compensated tunable filter calibration device for Bragg grating interrogation having a set of reference wavelengths, enabling accurate determination of Bragg grating wavelengths. There are two devices, first is a system that estimates the temperature of an array of gratings using an array of gratings bonded to a common host substrate and a single grating bonded to a material with different linear coefficient of thermal expansion, this is called a dual-substrate Bragg grating calibration system. Changes in a common temperature of the subsstrates is measured by monitoring the difference betweeen shifts of grating wavelength. As a filter is scanned from its lowest to highest voltage and the voltages are recorded. The second lowest wavelength corresponds to the grating attached to the differing substrate. The voltages are used to calculate the voltage-to-wavelength function for the scanning range of the filter. To compensate for variations in a calibration curve and temperature variations of the calibration array, the temperature is estimated and function recalculated at every pass of the scanning filter.
The second system uses a wavelength reference absorption cell, preferably a hydrogen-cyanide (H14C13N) type of wavelength reference absorption cell, that absorbs light at discrete wavelengths corresponding to the molecular vibrational mode frequencies of the gas. A wavelength reference absorption cell utilizing acetylene may be used, however the hydrogen-cyanode cell has more lines across a bigger range. With a broadband input to the cell, the output displays the spectrum of the input with several narrow dips in the spectrum corresponding to the absorption lines. A photodetector sees the transmission spectrum of the absorption cell while another photodetector sees the Bragg gratings reflection from a sensing array. The filter drive voltages that coincide with the dips of the transmission spectrum are used to calibrate the voltage-to-wavelength function of the scanning filter. In this system, there is no temperature compensation step as the absorption lines are not sensitive to temperature.